1. Field of the Invention
The invention is a solar energy collection tube, per se, for use in a solar energy utilization system. There is no specific U.S. classification for such inventions, but the prior art is found chiefly in U.S. Class 126/270 Solar Heaters and U.S. Class 126/271 Solar Water Heaters.
2. Description of the Prior Art
It is well known that a fluid may be heated by circulating it through a blackened metal pipe that is exposed to the sun's rays. In addition to conducting heat into the circulating fluid for some useful purpose, the metal pipe or solar energy collecting tube as it may be called, tends to dissipate its heat by radiation to the surroundings and by conduction to the medium in contact with its outside surface (generally air), the conductive heat loss being exacerbated by convection currents. The prospect of avoiding this latter heat loss has induced many inventors to seek means of enclosing the collector tube in a transparent evacuated jacket.
Solar energy collector tubes having flow of a heat transfer fluid into one end of a metal tube and out of the opposite end of the tube, with a transparent jacket supported around the metal tube by means of spacers and with provision for evacuation of the free space intermediate the tubes, are disclosed in the following U.S. Patents:
U.s. pat. No. 1,855,815 -- 4/1932 -- Abbot PA1 U.s. pat. No. 1,946,184 -- 2/1934 -- Abbot PA1 U.s. pat. No. 1,989,999 -- 2/1935 -- Niederle PA1 U.s. pat. No. 3,227,153 -- 1/1966 -- Godel et al. PA1 U.s. pat. No. 3,915,147 -- 10/1975 -- Rineer PA1 U.s. pat. No. 3,952,724 -- 4/1976 -- Pei PA1 U.s. pat. No. 3,983,861 -- 10/1976 -- Beauchaine PA1 U.s. pat. No. 4,002,160 -- 1/1977 -- Mather Jr., G. R. PA1 (1) Inefficiencies due to reflective loss and thermal lag. PA1 (2) Reduced heat transfer fluid flow capacity or increased pressure specifications. PA1 (3) Conductive heat loss by spacer element. PA1 (4) Complicated manifolding. PA1 (5) Technically difficult and expensive manufacture. PA1 (6) Fragility and vulnerability.
In such an assembly the inner tube tends to elongate differentially with respect to the outer jacket tube when the collector is put in service, due to the difference in coefficients of expansion of the two materials together with the fact that the inner tube tends to reach a higher temperature, so that a special problem is present in making the seals between the spacers and the tubes, so that the seals or tubes are not destroyed by the stresses set up in the assembly. In the 1932 patent to Abbot it is not apparent that the inventor was acquainted with this problem and the illustration of the seals does not suggest that the problem was solved. But in his 1934 patent, Abbot added a a metal bellows to the outer jacket, thereby introducing flexibility and supplying a solution to the differential elongation problem. The 1935 patent to Niederle does not acknowledge the problem and the illustrations do not suggest that his construction was operative in view of the problem.
In recent years, those who have sought to realize the improved efficiency of a vacuum jacketed collector tubes have used the Dewar flask or familiar vacuum bottle type construction, the now conventional approach which is seen in various forms in the following U.S. Patents:
In the above disclosures, the collecting tube is closed at one end and inserted into a glass tube also closed at one end, in the manner of placing a test tube inside a larger test tube. A spacing device of some form is employed to center the free end of the collecting tube and support it against inertial movement within the outer tube. At the open end of the collector tube, a hermetic "ring seal" is made and the space between the two tubes is evacuated in the factory via a temporary side tube which is then permanently sealed off. Note that a third tube must be included into the assembly, to carry the heat transfer fluid to the blind end of the collector tube for circulation, and that inlet and outlet tubes must be specially provided.
Such construction has the following disadvantages: (1) Introduction of the third tube entails inefficiencies due to (a) reflective radiant energy loss from the surfaces of the intermediate tube and (b) lag in heat transfer through the wall of the intermediate tube which is commonly of glass, a poor thermal conductor. (2) Since flow must enter and exit from the same end of the tube, the effective cross sectional area of the heat transfer fluid conduit is reduced by half for any given collector tube diameter, requiring (a) increased working pressure of the heat transfer medium and (b) increased performance specifications of the system, to withstand the higher working pressure. (3) The spacer element separating and supporting the intermediate tube within the outer jacket affords a path for conductive heat loss (albeit a small one) from the inner collector tube to the outer jacket. (4) Solar energy utilization systems generally employ multiple collection tubes arranged in parallel. The accessory manifolds for plumbing these connections are complex in design, since inlets and outlets are in close proximity and in some examples arranged concentrically (cf. U.S. Pat. No. 3,952,724 4/76 Pei). ( 5) The collector tube assembly is of complex design, having a ring seal, an inlet port, an outlet port and sometimes a graded metal to glass seal in close proximity (cf. U.S. Pat. No. 3,227,153 1/1966 Godel). Fabrication of such complex glassware is technically difficult and requires well-equipped glass manufacturer's facilities employed in the hands of skilled glass workers, and resulting in high unit costs. (6) The collector tube construction is inherently fragile by reason of its complexity and it is ill adapted for use in the hostile environment outdoors where it will be exposed to wind, rain, sleet, snow, ice and hail as well as the vandal's missiles. When a single collection tube in an installation is damaged, the entire system is disabled, and if a special heat transfer medium is employed, loss of the fluid will occur. In summary, the disadvantages of such systems include:
The primary problem with vacuum jacketing a solar energy tube, the problem prior inventors have solved by use of bellows or by single-ended vacuum bottle type construction is that if one puts a vacuum jacket on a collector tube by the conventional means -- typically, by means of "ring seals" at each end of the familiar straight through tube construction, the seals are subjected to great stresses when the inner tube elongates due to thermal expansion when it is placed in operation. The outer jacket, remaining relatively cold, does not elongate and is stressed in tension, resulting in fracture of the ring seals and loss of vacuum.
A second underlying problem is a methodological or philosophical one, the belief ingrained in contemporary practitioners of the inventive arts in solar energy, that any vacuum jacket must be evacuated just once in the factory and then sealed for use in the field.